The invention relates to a method and to a system for the non-invasive determination of the temperature on biological tissue treated by means of radiation, particularly laser radiation.
It is known to determine the tissue temperature in the thermal treatment of biological tissue by means of optoacoustic techniques (K. V. Larin, I. V. Larina, M. Motamedi, R. O. Esenaliev, xe2x80x9cMonitoring Temperature Distribution with an Optoacoustic Technique in Real Timexe2x80x9d, SPIE Proc. 3916: 311-321, 2000).
It is known from German Patent Document DE 199 32 477 A1 to determine by means of optoacoustic techniques material changes caused on biological tissue by pulsed irradiation.
From G. Schxc3xcle, G. Hxc3xcttmann, J. Roider, C. Wirbelauer, R. Birngruber, R. Brinkmann, xe2x80x9cOptoacoustic Measurement during xcexcs-Irradiation of the Retinal Pigment Epithelium, SPIE Proc. Vol. 3914: 213-236, 200, it is known to measure the temperatures at the treated ocular fundus during the selective microphoto coagulation at the ocular fundus for the treatment of diseases of the retina by means of xcexcs laser pulses. The pressure amplitude caused by the first treatment pulse was used for standardizing the pressure amplitude to the temperature of the ocular fundus. From the pressure amplitude increase of the subsequent treatment pulses, the temperature increase and, in addition, the respective absolute temperatures were determined by means of the dependence of the temperature on the pressure amplitudes known from calibration measurements.
In many spheres of ophthalmology, different energy sources, particularly lasers, are used for diagnostics and treatment. As a rule, the entire beamed-in energy is absorbed by the biological tissue and converted to heat, the resulting temperature increase achieving the desired treatment effect. For example, during laser photocoagulation, the retina of the eye is thermally coagulated in a targeted manner. In the case of the conventional irradiations with irradiation times about 100 ms, temperatures occur of above 60xc2x0 C. Also, in the case of transpupillary thermotherapy (TTT), temperatures increases are utilized for achieving a vascular occlusion. In the case of photodynamic therapy (PDT), a previously injected dye is activated by laser irradiation on the ocular fundus. The active ingredient develops its effect only on those cells to which to which it is bound. In this case also, almost the entire beamed-in energy is absorbed in the dye and in the retina and is converted to heat. During the respective irradiation time (pulse duration) with relatively long treatment and radiation pulses in the order of from xcexcs to several hundred seconds, a temperature increase of the treated biological tissue, particularly of the fundus of the eye, may occur which results in unintended damage to regions of the retina. A non-invasive real-time temperature determination could not yet be carried out in eye treatments of this type.
It is therefore an object of the invention to provide a method and a system for the non-invasive determination of the temperature on treated biological tissue, particularly in ophthalmology, in which radiation pulses are used for the treatment radiation.
With respect to the method, this object is achieved according to the invention by means of the characteristics of claim 1 and, with respect to the system, the object is achieved by means of the characteristics of claims 8 and 9.
According to the invention, during the respective irradiation period of the treatment radiation, additional radiation pulses of a shorter pulse duration and a lower energy than during the treatment radiation are aimed at the treated biological tissue, or the treatment radiation is switched off for a short time and is switched back on again. The additional radiation pulses or the short-duration switch-offs of the treatment radiation may take place essentially at the same time intervals. The thermal tissue expansions occurring when the additional radiation pulses are used and the tissue contractions occurring during the short-duration switch-offs of the treatment radiation are detected by a pressure measurement or by an optical measurement. From the respective measuring signals, which are caused by the additional radiation pulses (measuring radiation pulses) or the short-duration switch-offs of the treatment radiation, the temperature increase is determined; particularly the respective absolute values of the temperature are determined.
In the case of the invention, the pulse energy of the additionally beamed-in radiation pulses or the switch-off time, during which the treatment radiation is switched off several times, will be constant. The energy applied into the tissue causes a temperature-dependent, optoacoustically analyzable, thermal expansion of the tissue, for which the green iron coefficient is a measurement. By means of calibration measurements, a calibration curve is obtained which has a linear rise of the acoustic amplitudes with the temperature according to the green iron coefficient (G. Schxc3xcle, G. Hxc3xcttmann, J. Roider, C. Wirbelauer, R. Birngruber, R. Brinkmann, xe2x80x9cOptoacoustic Measurement during xcexcs Irradiation of the Retinal Pigment Epithelium, SPIE, Proc., Vol. 3914: 230-236, 2000).
In the case of the invention, short-duration laser pulses are preferably used with a few ns, for example 8 ns, and with a low pulse energy of a few xcexcJ, for example 5 xcexcJ. The laser pulses may also be longer. Because of the high light absorption at the fundus of the eye, particularly of the retinal pigmentary epithelium (RPE), the low pulse energies are sufficient for obtaining sufficient measuring values, particularly of the pressure measurement (acoustic measurement) and of the optical measurement. In the case of the pressure measurement, preferably a maximal value or an integral of the respectively measured pressure half-wave is used for the analysis during the temperature determination. However, other algorithms, such as the slope or a Fourier transformation of the measuring signals, can also be used in the analysis.
As an optical measurement, an interference measurement by means of an interferometer is preferably used, particularly a fiber interferometer whose measuring beam is coupled into the treatment lens system of the treatment radiation.
Preferably, during the switch-on of the treatment radiation by means of an additional short-duration radiation pulse or immediately after the switch-on of the treatment radiation by means of a short-duration switch-off, a measuring signal is obtained from the occurring tissue expansion or contraction, which measuring signal is standardized to the normal body temperature, for example, the human body temperature of 37xc2x0 C.
On the basis of the continuous monitoring of the temperature also during the respective irradiation time (pulse duration) of the treatment radiation, a control of the treatment radiation can be carried out as a function of the respective determined temperature at the irradiation site. In this case, particularly the pulse duration or pulse power of the treatment beam can be controlled in order to obtain the desired temperature at the irradiation site.